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How Airbags Inflate

An airbag is a rapidly-inflating cushion that helps to prevent auto-crash injuries. When there is a sudden deceleration of the auto, a switch is thrown which triggers the start of a chemical reaction that produces a gas. The chemical reaction employed is the thermal decomposition of sodium azide, NaN3.
Upon heating, sodium azide decomposes rapidly to form sodium metal and nitrogen gas. The reaction is exothermic and extremely rapid once it has begun. In practice an oxidant is added to consume the sodium metal produced in the reaction. When the decomposition is triggered by the sudden application of heat, the hot nitrogen gas produced expands into the airbag. Expansion occurs in only about 50 milliseconds. After the initial expansion, the hot nitrogen gas cools, causing the bag to shrink and freeing the passenger.
Note that no new substance has been introduced into this closed system during the reaction. The large volume difference between reactants and products is due to the fact that one of the products is a gas.